Gravity in reverse: the tale of Albert Einstein's "greatest blunder"

In effect, GR accounts for two opposite phenomena: good ol' gravity, such as the attraction between the Earth and a ball thrown into the air or between the Sun and the Earth; and a mysterious, repulsive pressure associated with the vacuum of space-time itself. Acting against gravity, lambda preserved what Einstein and every other physicist of his day had strongly believed in: the status quo of a static universe. Static it was, but stable it was not. And to invoke an unstable condition as the natural state of a physical system violates scientific credo: you cannot assert that the entire universe is a special case that happens to be precariously balanced for eternity. Nothing ever seen, heard, or measured has acted that way in the history of science. Yet, in spite of being deeply uneasy with lambda, Einstein included it in his equations.

Twelve years later, in 1929, the U.S. astronomer Edwin P. Hubble discovered that the universe is not static after all: convincing evidence showed that the more distant a galaxy, the faster that galaxy is receding from the Earth. In other words, the universe is growing. Embarrassed by lambda, and exasperated by having thus blown the chance to predict the expanding universe himself, Einstein discarded lambda, calling its introduction his life's "greatest blunder."

That wasn't the end of the story, though. Off and on over the decades, theoreticians would exhume lambda--more commonly known as the "cosmological constant"--from the graveyard of discredited theories. Then, sixty-nine years later, in 1998, science exhumed lambda one last time, because now there was evidence to justify it. Early that year two teams of astrophysicists--one led by Saul Perlmutter of Lawrence Berkeley National Laboratory in Berkeley, California; the other by Brian Schmidt of Mount Stromlo and Siding Springs Observatories in Canberra, Australia--made the same remarkable announcement. Dozens of the most distant supernovas ever observed, they said, appeared noticeably dimmer than expected--a disturbing finding, given the well-documented behavior of this species of exploding star. Reconciliation required that either those distant supernovas acted quite differently from their nearer brethren, or else they were as much as 15 percent farther away than the prevailing cosmological models had placed them.

Not only was the cosmos expanding, but a repulsive pressure within the vacuum of space was also causing the expansion to accelerate. Something had to be driving the universe outward at an ever-increasing pace. The only thing that "naturally" accounted for the acceleration was lambda, the cosmological constant. When physicists dusted it off and put it back in Einstein's original equations for general relativity, the state of the universe matched the state of Einstein's equations.

To an astrophysicist, the supernovas used in Perlmutter's and Schmidt's studies are worth their weight in fusionable nuclei. Each star explodes the same way, igniting a similar amount of fuel, releasing a similarly titanic amount of energy in a similar period of time, and therefore achieving a similar peak luminosity. Hence these exploding stars serve as a kind of yardstick, or "standard candle," for calculating cosmic distances to the galaxies in which they explode, out to the farthest reaches of the universe.